The present invention is a peak-detecting demodulator for the conversion of information signals in a.c. format to d.c. voltages representing the information.
In various areas of the field of electronics it is advantageous and sometimes necessary to convert a.c. signals to d.c. voltages. One such area to which the present invention can be applied is the field of stabilization systems. Therein, the source is generally a synchro, resolver, or Rate Integrating Gyro (RIG) pick-off. Taking the resolver as an example, it can be thought of as a rotating transformer whose output varies as a function of shaft angle. The variation ranges from a peak amplitude in-phase with the primary reference excitation to a peak which is 180.degree. out of phase with respect to the reference. The present invention accepts these variable amplitude signals, and with utilization of the reference excitation, provides a d.c. type signal representing the peak amplitude of the a.c. input that will range from a maximum positive peak for input signals in phase with the reference to a maximum negative peak for signals out of phase.
It has always been desirable in stabilization systems to have a "stiff" but wide band loop. Some of the design limitation has been due to the existing demodulator designs for converting a.c. to d.c. signals necessary to drive d.c. torque motors used on the platforms to be stabilized. These demodulators were generally some form of a synchronous full-wave rectifier with a low-pass filter. The low-pass filter was intended to reduce the output ripple to a acceptable level, and was usually a problem area that limited the frequency response of the system.
The present invention overcomes the problems of the above-described, prior systems, and does so in part, by replacing the typical low-pass filter with a sample-and-hold circuit. By sampling the peak information of the input a.c. signal and storing it until the next peak sample, a d.c. output of extremely low ripple is achieved. The response of the present invention allows the output to change amplitude as fast as twice the input frequency (or 1/2 the period of the input).
Primary advantages of the present invention over prior devices include that samples are taken at the peaks of the signal, and that the reference signal which drives the rectifier and sample trigger is always at some finite value that is large enough to avoid failures in generating trigger or switch commands regardless of how near the input to be processed is to zero, such as is the case when the shaft angle of a resolver is near zero degrees. Also, the present invention provides faster response than does a low pass filter. If the filter is modified to increase its rate of response, ripple also increases, whereas the present invention maintains a low ripple factor by sampling only the peaks of the signal.